Reproduction process using infrared radiation

ABSTRACT

A method is disclosed of making an image on a layer, which comprises exposing a layer of polyethylene to infrared radiation in heat-conductive relation with a master to form a latent image, charging the layer with an electrostatic charge at a substantially later time to produce a charge image corresponding to the latent image, developing the charge image with a toner, and fusing the toner to said layer. A method of making an offset printing plate is also disclosed which comprises exposing a layer of cellulose acetate to infrared radiation in heat-conductive relation with a master to form a latent image corresponding to the master, charging the layer electrostatically at a substantially later time to produce a charge image, developing the charge image by applying a toner, fusing the toner to said layer and saponifying the surface of said layer to render the non-image areas hydrophilic thereby forming a layer suitable for use as an offset printing plate.

United States Patent 1 Lind [451 May a, 1973 [54] REPRODUCTION PROCESS USING [73] Assignee: Kalle Aktieng esellschatt,

Wiesbaden-Biebrich, Germany 22 Filed: Nov.6, 1961 21 Appl.No.: 150,141

[30] Foreign Application Priority Data Dec. 17, 1960 Germany ..K 42434 [52] US. Cl. ..2S0/65 T, 117/l7.5, 101/463 [51] Int. Cl. ..G0ln 21/34 [58] Field of Search ..l0l/149.2, 426 CR,

101/426 ES; 250/651, 49.5, 65.2, 65 T; 96/1 PC; 1l7/l7.5

[56] References Cited UNITED STATES PATENTS 9/1957 Australia ..l01/ES OTHER PUBLICATIONS Prater-Superthreshold Impact Heat Printing. Pub. in I.B.M. Technical Disclosure Bulletin, Vol. 1, No. 6, Apr. 1959, page 4.

Australia ..101/ES Primary Examiner-Edgar S. Burr Attorney-James E. Bryan [57] ABSTRACT A method is disclosed of making an image on a layer, which comprises exposing a layer of polyethylene to infrared radiation in heat-conductive relation with a master to form a latent image, charging the layer with an electrostatic charge at a substantially later time to produce a charge image corresponding to the latent image, developing the charge image with a toner, and fusing the toner to said layer. A method of making an offset printing plate is also disclosed which comprises exposing a layer of cellulose acetate to infrared radiation in heat-conductive relation with a master to form a latent image corresponding to the master, charging the layer electrostatically at a substantially later time to produce a charge image, developing the charge image by applying a toner, fusing the toner to said layer and saponifying the surface of said layer to render the non-image areas hydrophilic thereby forming a layer suitable for use as an offset printing plate.

2 Claims, No Drawings REPRODUCTION PROCESS USING INFRARED RADIATION It is known that changes in the temperature of a material are accompanied by changes in the specific electrical resistance thereof. Resins of high molecular weight and of greatly differing composition undergo a loss of electrical resistance amounting to several powers of when heated from room temperature to about 100C.

Unexpectedly, it has been found that these resins of high molecular weight remain in a state of lowered resistance for a period of time after heating has ceased, even when the temperature thereof has been reduced below the treatment temperature or even down to the initial temperature. The present invention utilizes this property.

The present invention relates to an image-recording process by image-wise heat action on a resin coating which undergoes loss of specific electrical resistance under the influence of heat, in which the resin coating is exposed image-wise to heat, then electrostatically charged and the latent electrostatic image is made visible. When required, the image is fixed by methodsknown in electrophotography.

The image-wise action of heat on the resin coating can be effected in various ways, e.g., the coating is exposed image-wise to heat radiation by means of an optical system or it is contacted with a heated object having the form of the image or it is placed under a master and exposed to radiant heat so that the heat is absorbed primarily in the image parts of the master and transferred by conduction to the resin coating. The heat action can thus be effected either by a contact process or by absorption of rays in the resin coating.

As heat sources, heated members of the usual type can be used, e.g., commercially available radiators. Where radiant heat is used for image-wise production of-heat, the temperature of these radiators is in the range of from about 400' to about 2,500C, preferably 1,500 2,000C. Also, heated stamps can be used.

In the preferentially heated image parts, loss of specific electrical resistance occurs. The heat can be continued to the melting point of the coating, or in excess thereof, but generally a considerably lower degree of heating is sufficient for obtaining an adequate effect. In many cases an increase in the temperature of the image parts, above room temperature, of 150C, preferably about 100C or less, is sufficient to ensure adequate differentiation.

After the completion of image-wise heat treatment, the resin coating is reduced to a temperature lower than that of the image-wise treatment. This can be done in various ways, e.g., by removal of the heat source so that the resin coating cools of its own accord to a lower temperature. Also, rapid cooling by the introduction of the coating into a refrigerating device gives good results. The coating is advantageously cooled to the initial starting temperature, i.e., the temperature it had before the heat treatment, but temperatures both higher and lower than this can be employed. In this way, a latent electrical resistance image, in which the image parts have lower electrical resistance than the image-free parts, is produced on the resin coating.

Suitable resins for use in the invention are those in which the specific resistance under normal conditions is within the range of about 10 ohm-cm. to about 10" ohm-cm. and in which the specific resistance in the surface area in contact with infra-red absorbing image parts of the master is reduced by at least two powers of 10 under the influence of heat. The value of this reduced specific resistance is preferably under 10 to 10 ohm-cm.

The suitability of a resin coating for the process of the invention may be ascertained, for example, as follows. The current flow resistance of the coating is evaluated as a function of temperature, whereupon it will be apparent whether there is adequate loss of resistance with increase in temperature. The loss in resistance of the resin coating that is applied to the support can, in the case of some resins, be improved by the addition of plasticizers to the resin tricresyl phosphate is for example suitable in the case of polystyrene so that there are very favorable resistance characteristics.

The suitability of a resin for the process of the invention can also be very simply assessed by the following test: A solution of the resin is poured upon a supporting material, e.g., a paper foil; the solvent is driven off in a current of air of moderate temperature; a master is placed on the coating and radiant heat is briefly beamed upon the master. The coating is then charged by a corona discharge and the charged coating is contacted with an electroscopic powder. If there is adequate differentiation in the resistance at the temperature employed, an image of the master will be obtained.

Resins which may be used in the invention under the conditions specified above include both synthetic and natural resins, e.g., polyolefins such as polyethylene, polyisobutylene, polypropylene and polybutylene, polyvinyl compounds such as polyvinyl chloride, chlorinated polyvinyl chloride, polyvinyl acetate, polystyrene particularly modified polystyrene interpolymers and polymer mixtures of the compounds already mentioned, polyesters such as polyterephthalic acid ester, cellulose esters such as cellulose acetate and cellulose propionate, maleinate resins, coumarone resins and ketone resins.

Of the natural resins, shellac, for example, has properties which make the use thereof in the present invention particularly advantageous. Also, conversion products of natural resins, e.g., polymerized resins, are very suitable.

The resins on which the resistance images are produced may be used as such, in self-supporting form, or they may be applied to a support, e.g., in known manner in the form of a melt or from solvents or in the form of dispersions. The thickness of the coated resin layer may vary within wide limits. For example, coatings of between 10 and 30 u may be applied. It is also possible for coatings that are thinner or thicker than this to be used. If it is desired for the resistance images to be made visible in the manner to be described below it is often advantageous for coatings of not less than 5 y. to be selected so that disruptive discharge will not occur when the electrostatic charge is applied. For the supporting material, many different substances are suitable, e.g., paper, plastics, metals, pottery, and glass. Foils made of these materials are used with particular advantage.

The latent electrical resistance images described above can be converted into a visible image. For this purpose, the resistance images are charged by means of a corona discharge. The charge is retained in the parts that have not been heated, while in the heated parts, which have a considerably lower resistance, the charge leaks away, at least partially, so that an electrostatic image corresponding to the resistance image is obtained and which is converted in known manner, by being contacted with a finely divided pigmented resin or carbon black particles, into a visible image which can be fixed by heat or treatment with a solvent vapor. In this case, the resins are advantageously applied to supports which possess adequate electrical conductivity, e.g., foils or plates made of metal, glass plates of which the surface has been made electrically conductive, and paper, plates or foils made of electrically conductive, non-fibrous material (plastics, resins). Materials which possess adequate electrical conductivity for use in the present invention are those with a specific resistance of l ohm-cm. at the most. If paper is used as a support for the coating which becomes conductive when heated, it is recommended that the paper he treated beforehand to prevent penetration of the coating solution, e.g., with methyl cellulose in aqueous solution or polyvinyl alcohol in aqueous solution or with a solution of an interpolymer of acrylic acid methyl ester and acrylonitrile in acetone and methyl-ethyl-ketone or with solutions of polyamides in aqueous alcohols. Also, aqueous dispersions of materials suitable for the pretreatment of the paper surface can be used.

If, as the resin, cellulose acetate is used on a waterresistant supporting material, after the processes already described of image-production and fixing of the image parts by heat treatment, the cellulose acetate can be superficially saponified in the image-free parts by means e.g., of alcoholic potassium hydroxide so that it is hydrophilic. Printing plates suitable for planographic printing are thus produced.

With transparent supporting material, intermediate originals for further copying processes on any type of light sensitive material can be obtained.

If the resistance images are to be used for recording purposes, it must be ensured that the resin coating absorbs heat rays adequately. For this purpose, a material such as carbon black, which'abs'orbs infra-red rays, can be incorporated in the coating or a suitably absorbent coating can be placed in contact'therewith. The resin coating can, for example, be placed on infra-red absorbent supporting material, e.g., black pigmented paper. The resistance images obtained by the heat treatment can then be made visible by electrostatic charging and development with a white or otherwise pigmented resin powder. Colorless material can also be used if it is closely contacted with an infra-red absorbent substance during the irradiation with infra-red rays. For this purpose, the recording material may, for example, be run over a blackened roller while irradiation is performed or it can be brought, during this process, into close contact with a blackened film, e.g., a blackened silver film.

The invention will be further illustrated by reference to the following specific examples:

EXAMPLE I A base paper with a weight of grams/sq. meter is coated by extrusion with polyethylene so that the coating thickness is about 15a and a master with text on one side only is placed on the polyethylene coating. The copying material arranged in this way is passed in close contact, at a speed of several meters a minute, past a focussed 1,350 watt infra-red radiator. As a result of infra-red absorption in the master, a heat image is formed which is transferred to the polyethylene coating as a result of the close contact. In the image parts, the specific electrical resistance of the polyethylene coating drops and a resistance differentiation is obtained which corresponds to the image parts of the master. To make this latent resistance image visible, the polyethylene coating is positively charged, by means of a corona discharge, to a surface potential of about 300 volts above ground. The latent resistance image then changes into a latent charge image, which is negative with respect to the master, because the charge immediately leaks away in the low resistance parts produced by the transfer of the heat image. Only the unheated areas corresponding the image-free parts of the original retain the electrostatic charge. The electrostatic charge image, which is a negative of the original, is made visible by means of a developer which may consist, for example, of parts by weight of glass balls ofa grain size of 300 400 p. and 2.5 parts by weight of a resin powder of a grain size of 20 50 M. The resin powder is obtained by melting together 30 parts by weight of polystyrene, 30 parts by weight of resin-modified maleic acid resin (Beckacite K and 3 parts by weight of carbon black (Peerless Black Russ 552), after which the melt is ground and screened.

EXAMPLE ll A wax paper of a weight of 40 grams/sq. meter is mechanically coated on both sides with a solution of 10 parts by weight of polyvinyl chloride in 75 parts by volume of ethyl acetate. After the coating has dried, the reproduction material is placed upon a master, with text on one or both sides, and infra-red rays are beamed upon the side remote from the master. A resistance differentiation is obtained on the surface of the polyvinyl chloride coating which corresponds to the image parts of the master and which is adequately maintained for up to an hour, so that even after that period has elapsed, conversion into an electrostatic image, as described in detail in Example 1, is still possible. The charge image is made visible in known manner by dusting over with a pigmented resin powder.

EXAMPLE [II A cellulose acetate foil of a thickness of 30 [L is laminated to a paper of I40 grams/sq. meter that has been made water-resistant by treatment with melamine resin. The cellulose acetate coating is exposed under a film master to a brief intensive infra-red radiation. The latent image formed on the acetate coating is made visible if the acetate coating is positively charged, by means of a corona discharge, and the resultant charge image is developed with the developer made up of glass balls and resin powder described in Example 1. After the powder image has been fixed by brief heating, the

resultant copy can be converted into a planographic printing plate. For this purpose, the image-bearing acetate coating is wiped over with a methanol solution of potassium hydroxide obtained by dissolving 12 parts by weight of potassium hydroxide in 90 parts by volume of methanol, the solution then being made up with water to 150 parts by volume. The acetate film is saponified in the image-free parts by the methanolic potassium hydroxide while the image parts, consisting of the fixed resin powder, are unaffected. The imagefree parts are water receptive, in contradistinction to the image parts which can be linked up with greasy ink. This paper printing foil can be used for printing in a planographic apparatus.

EXAMPLE IV A solution of 22 parts by weight of a maleinate resin, the resin having a softening temperature of 95 105C, in 200 parts by volume of ethyl alcohol is applied mechanically to a paper of a weight of 110 grams/sq. meter that has been precoated to prevent the penetration of organic solvents. After the coating has dried, the reproduction material is exposed behind a positive film master to brief irradiation with infrared. To make the latent image visible, the maleinate resin coating is positively charged by means of a corona discharge. The electrostatic charge is retained only in those parts of the reproduction coating that have not received heat from the original. A negative charge image of the original is obtained which is made visible by treatment with the developer made up of glass balls and a pigmented resin powder described in Example 1. Fixing of the powder image is effected by a brief treatment with trichloroethylene vapor.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is: l. A method of making an image on a layer, which comprises the steps of:

exposing a layer of polyethylene to infrared radiation in heat conductive relation with a master to form thereon a permanent, developable, latent image corresponding to the master image; charging said layer with an electrostatic charge at a substantially later time to produce a charge image corresponding to the latent image; developing the charge image with a toner which selectively adheres to said layer to render the charge image; and fusing the toner to said layer to produce a permanent image. 2. A method of making an offset printing plate, which comprises the steps of:

exposing a layer of cellulose acetate to infrared radiation in heat-conductive relation with a master to form thereon a permanent, developable, latent image corresponding to the master image; charging the layer electrostatically at a substantially later time to produce a charge image corresponding to the latent image; developing the charge image on the layer by applying a toner which selectively adheres to said layer;

fusing th e toner to said la er; and sapomfymg the surface 0 said layer with an alcohol- 

2. A method of making an offset printing plate, which comprises the steps of: exposing a layer of cellulose acetate to infrared radiation in heat-conductive relation with a master to form thereon a permanent, developable, latent image corresponding to the master image; charging the layer electrostatically at a substantially later time to produce a charge image corresponding to the latent image; developing the charge image on the layer by applying a toner which selectively adheres to said layer; fusing the toner to said layer; and saponifying the surface of said layer with an alcoholic solution of potash to render the non-image areas hydrophilic to form a layer suitable for use as an offset printing plate. 